Inheritance

To avoid redefining attributes and methods for similar classes, we can write a single base class from which the similar classes inherit. For example, we can write a class called Pet and define Dog as a subclass of Pet:

class Pet:

    def __init__(self, name, owner):
        self.is_alive = True    # It's alive!!!
        self.name = name
        self.owner = owner

    def eat(self, thing):
        print(self.name + " ate a " + str(thing) + "!")

    def talk(self):
        print(self.name)

class Dog(Pet):

    def talk(self):
        super().talk()
        print('This Dog says woof!')

Inheritance represents a hierarchical relationship between two or more classes where one class is a more specific version of the other: a dog is a pet (We use is a to describe this sort of relationship in OOP languages, and not to refer to the Python is operator).

Since Dog inherits from Pet, the Dog class will also inherit the Pet class's methods, so we don't have to redefine __init__ or eat. We do want each Dog to talk in a Dog-specific way, so we can override the talk method.

We can use super() to refer to the superclass of self, and access any superclass methods as if we were an instance of the superclass. For example, super().talk() in the Dog class will call the talk() method from the Pet class, but passing the Dog instance as the self.

This is a little bit of a simplification, and if you're interested you can read more in the Python documentation on super.

Q1: That's inheritance, init?

Let's say we want to create a class Monarch that inherits from another class, Butterfly. We've partially written an __init__ method for Monarch. For each of the following options, state whether it would correctly complete the method so that every instance of Monarch has all of the instance attributes of a Butterfly instance. You may assume that a monarch butterfly has the default value of 2 wings.

class Butterfly():
    def __init__(self, wings=2):
        self.wings = wings

class Monarch(Butterfly):
    def __init__(self):
        _________________________________________
        self.colors = ['orange', 'black', 'white']

super.__init__()

super().__init__()

Butterfly.__init__()

Butterfly.__init__(self)

Some butterflies like the Owl Butterfly have adaptations that allow them to mimic other animals with their wing patterns. Let's write a class for these MimicButterflies. In addition to all of the instance variables of a regular Butterfly instance, these should also have an instance variable mimic_animal describing the name of the animal they mimic. Fill in the blanks in the lines below to create this class.

class MimicButterfly(______________):
    def __init__(self, mimic_animal):
        _______________.__init__()
        ______________ = mimic_animal

What expression completes the first blank? Your Answer:

What expression completes the second blank? Your Answer:

What expression completes the third blank? Your Answer:

Q2: Shapes

Fill out the skeleton below for a set of classes used to describe geometric shapes. Each class has an area and a perimeter method, but the implementation of those methods is slightly different. Please override the base Shape class's methods where necessary so that we can accurately calculate the perimeters and areas of our shapes with ease.

import math
pi = math.pi

class Shape:
    """All geometric shapes will inherit from this Shape class."""
    def __init__(self, name):
        self.name = name

    def area(self):
        """Returns the area of a shape"""
        print("Override this method in ", type(self))

    def perimeter(self):
        """Returns the perimeter of a shape"""
        print("Override this function in ", type(self))

class Circle(Shape):
    """A circle is characterized by its radii"""
    def __init__(self, name, radius):
        super().__init__(name)
        self.radius = radius

    def perimeter(self):
        """Returns the perimeter of a circle (2πr)"""
        return 2*pi*self.radius

    def area(self):
        """Returns the area of a circle (πr^2)"""
        return pi*self.radius**2

class RegPolygon(Shape):
    """A regular polygon is defined as a shape whose angles and side lengths are all the same.
    This means the perimeter is easy to calculate. The area can also be done, but it's more inconvenient."""
    def __init__(self, name, num_sides, side_length):
        super().__init__(name)
        self.num_sides = num_sides
        self.side_length = side_length

    def perimeter(self):
        """Returns the perimeter of a regular polygon (the number of sides multiplied by side length)"""
        return self.num_sides*self.side_length
class Square(RegPolygon):
    def __init__(self, name, side_length):
        super().__init__(name, 4, side_length)

    def area(self):
        """Returns the area of a square (squared side length)"""
        return self.side_length**2

class Triangle(RegPolygon):
    """An equilateral triangle"""
    def __init__(self, name, side_length):
        super().__init__(name, 3, side_length)

    def area(self):
        """Returns the area of an equilateral triangle is (squared side length multiplied by the provided constant"""
        constant = math.sqrt(3)/4
        return constant*self.side_length**2

Q3: Cat

Below is a skeleton for the Cat class, which inherits from the Pet class we saw in the Inheritance introduction. To complete the implementation, override the __init__ and talk methods and add a new lose_life method.

Hint: You can call the __init__ method of Pet (the superclass of Cat) to set a cat's name and owner.

Hint: The __init__ method is not a real constructor, and can be called like any other method.

class Cat(Pet):

    def __init__(self, name, owner, lives=9):
        super().__init__(name, owner)
        self.lives = lives

    def talk(self):
        """Print out a cat's greeting.

        >>> Cat('Thomas', 'Tammy').talk()
        Thomas says meow!
        """
        print(self.name + ' says meow!')
    def lose_life(self):
        """Decrements a cat's life by 1. When lives reaches zero,
        is_alive becomes False. If this is called after lives has
        reached zero, print 'This cat has no more lives to lose.'
        """
        if self.lives > 0:
            self.lives -= 1
            if self.lives == 0:
                self.is_alive = False
        else:
            print("This cat has no more lives to lose.")
    def revive(self):
        """Revives a cat from the dead. The cat should now have 
        9 lives and is_alive should be true. Can only be called
        on a cat that is dead. If the cat isn't dead, print 
        'This cat still has lives to lose.'
        """
        if not self.is_alive:
            self.__init__(self.name, self.owner)
        else:
            print('This cat still has lives to lose.')

Q4: NoisyCat

More cats! Fill in this implementation of a class called NoisyCat, which is just like a normal Cat. However, NoisyCat talks a lot: in fact, it talks twice as much as a regular Cat! If you'd like to test your code, feel free to copy over your solution to the Cat class above.

class NoisyCat(Cat): # Fill me in!
    """A Cat that repeats things twice."""
    def __init__(self, name, owner, lives=9):
        # Is this method necessary? Why or why not?
        super().__init__(name, owner, lives)
        # No, this method is not necessary because NoisyCat already inherits Cat's __init__ method

    def talk(self):
        """Talks twice as much as a regular cat.
        >>> NoisyCat('Magic', 'James').talk()
        Magic says meow!
        Magic says meow!
        """
        super().talk()
        super().talk()

Representation: Repr, Str

There are two main ways to produce the "string" of an object in Python: str() and repr(). While the two are similar, they are used for different purposes.

str() is used to describe the object to the end user in a "Human-readable" form, while repr() can be thought of as a "Computer-readable" form mainly used for debugging and development.

When we define a class in Python, __str__ and __repr__ are both built-in methods for the class.

We can call those methods using the global built-in functions str(obj) or repr(obj) instead of dot notation, obj.__repr__() or obj.__str__().

In addition, the print() function calls the __str__ method of the object and displays the returned string with the quotations removed, while simply calling the object in interactive mode in the interpreter calls the _repr__ method and displays the returned string with the quotations removed.

Here are some examples:

class Rational:

    def __init__(self, numerator, denominator):
        self.numerator = numerator
        self.denominator = denominator

    def __str__(self):
        return f'{self.numerator}/{self.denominator}'

    def __repr__(self):
        return f'Rational({self.numerator},{self.denominator})'

>>> a = Rational(1, 2)
>>> str(a)
'1/2'
>>> repr(a)
'Rational(1,2)'
>>> print(a)
1/2
>>> a
Rational(1,2)

>>> s = "hello" # Python String objects also have __repr__ and __str__ methods!
>>> repr(s)
"'hello'"
>>> s
'hello' # displays the repr string with the outer layer of quotations removed
>>> print(repr(s))
'hello' # printing the repr string removes the outer layer of quotations
>>> str(s)
'hello'
>>> print(s)
hello # displays the str string with quotations removed

Q5: WWPD: Repr-esentation

Note: This is not the typical way repr is used, nor is this way of writing repr recommended, this problem is mainly just to make sure you understand how repr and str work.

class A:
    def __init__(self, x):
        self.x = x

    def __repr__(self):
         return self.x

    def __str__(self):
         return self.x * 2

class B:
    def __init__(self):
         print('boo!')
         self.a = []

    def add_a(self, a):
         self.a.append(a)

    def __repr__(self):
         print(len(self.a))
         ret = ''
         for a in self.a:
             ret += str(a)
         return ret

Given the above class definitions, what will the following lines output?

>>> A('one')

>>> print(A('one'))

>>> repr(A('two'))

>>> b = B()

>>> b.add_a(A('a'))
>>> b.add_a(A('b'))
>>> b

Q6: Cat Representation

Now let's implement the __str__ and __repr__ methods for the Cat class from earlier so that they exhibit the following behavior:

>>> cat = Cat("Felix", "Kevin")
>>> cat
Felix, 9 lives
>>> cat.lose_life()
>>> cat
Felix, 8 lives
>>> print(cat)
Felix

# (The rest of the Cat class is omitted here, but assume all methods from the Cat class above are implemented)
    def __repr__(self):
        return self.name + ", " + str(self.lives) + " lives"
    def __str__(self):
        return self.name